Modify shader via Angle of Shadow
If you use the Shader to RGB node combined with Greater Than node. You can differentiate between light and shadow to create a cell-shading effect.



Simply add a texture to the original Diffuse BSDF, to texture the transition between light and shadow.



This is how the effect looks in motion.



You can sharpen or smoothen the shadows edge by decreasing the lamps energy, increasing its radius and lowering the threshold of the greather than node.